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Tuesday, August 13, 2013

IMAP: Working with Folders


In yesterday's article, "IMAP: Downloading emails", we saw how to log into an email account using IMAP, select a folder (in this case the INBOX), and download an email. The INBOX is the only folder that is standard in IMAP and always exists, even if empty.

In IMAP, however, the INBOX does not need to be the only folder. RFC3501, which to date defines the latest IMAP standard called IMAP4rev1 (which you no doubt noticed in CAPABILITY response in yesterday's article), provides several commands used for working with folders.

You can create folders using the CREATE command, which is used as follows:

CREATE foldername

Use this command to create a few folders we can work with:

You can now view a list of all folders using the LIST command, as follows:

02 LIST "" "*"

The parameters to LIST allow you to list a subset of folders rather than all of them, but that's not important at this stage. The command as above will give you the full list:

You'll notice the Trash and Sent folders which we didn't create. These folders were actually created by Thunderbird when I set up my account in it. The desktop clients create certain folders used to store sent emails, deleted emails, etc. These folders aren't standard, and in fact clients don't always use the same conventions (e.g. Thunderbird uses a Sent folder, while Outlook creates a Sent Items folder instead).

Aside from the actual folder name, each line of the LIST response gives you some additional information about the folder. In this case it's telling us that the folders don't have any children, and what the hierarchy delimiter is (in this case the dot). This is because folders can actually have subfolders, and the hierarchy delimiter separates each folder from its child (e.g. folder.subfolder if the dot is the hierarchy delimiter). There's actually a special version of the LIST command that is used specifically to retrieve the hierarchy delimiter:

C: 0006 LIST "" ""
S: * LIST (\Noselect) "." ""

Note that not all servers necessarily use the dot as the hierarchy delimiter. In hMailServer, for example, it is possible to change it:

Let's try creating a folder with a subfolder:

C: 0007 CREATE Code.PHP
S: 0007 OK CREATE Completed

If you open Thunderbird, you can see the folder hierarchy. First, though, you'll need to right click on the account to the left and select Subscribe...:

Yup, because in IMAP there's this thing called folder subscription. You might have hundreds of folders, but you might only be interested in a few of them. So aside from the LIST command, there's the LSUB command which only lists subscribed folders:

C: 0008 LSUB "" "*"
S: * LSUB (\HasNoChildren) "." "INBOX"
S: * LSUB (\HasNoChildren) "." "Trash"
S: * LSUB (\HasNoChildren) "." "Sent"
S: 0008 OK LSUB completed

Notice how this list is shorter than the one we got with the LIST command. We can subscribe folders using the SUBSCRIBE command (which is what happens if you tick those checkboxes in Thunderbird):

C: 0009 SUBSCRIBE Personal
S: 0009 OK Subscribe completed

If we repeat the LSUB command, the list is now updated:

C: 0010 LSUB "" "*"
S: * LSUB (\HasNoChildren) "." "INBOX"
S: * LSUB (\HasNoChildren) "." "Trash"
S: * LSUB (\HasNoChildren) "." "Sent"
S: * LSUB (\HasNoChildren) "." "Personal"
S: 0010 OK LSUB completed

To unsubscribe a folder, just use the UNSUBSCRIBE command:

C: 0011 UNSUBSCRIBE Personal
S: 0011 OK Unsubscribe completed

If you try the LSUB now, it will give you the same result as before.

You can delete a folder completely using the DELETE command:

C: 0013 DELETE Work
S: 0013 OK Delete completed

What are you looking at... everyone wants to delete work, no? :)

Finally, if you want to work with the emails in a folder (download them, flag them, mark them as unread, etc), then you'll have to select the folder. We've already seen the SELECT command in yesterday's article:

C: 0014 SELECT Personal
S: * FLAGS (\Deleted \Seen \Draft \Answered \Flagged)
S: * OK [UIDVALIDITY 1376393234] current uidvalidity
S: * OK [UIDNEXT 1] next uid
S: * OK [PERMANENTFLAGS (\Deleted \Seen \Draft \Answered \Flagged)] limited
S: 0014 OK [READ-WRITE] SELECT completed

We'll learn about the details of this response in another article. There is, however, another command that can be used to select folders: the EXAMINE command. Let's try it out:

C: 0018 EXAMINE Personal
S: * FLAGS (\Deleted \Seen \Draft \Answered \Flagged)
S: * OK [UIDVALIDITY 1376393234] current uidvalidity
S: * OK [UIDNEXT 1] next uid
S: * OK [PERMANENTFLAGS ()] limited
S: 0018 OK [READ-ONLY] EXAMINE completed

You'll notice that the response is mostly the same, except for the last two lines. The important thing to notice is that the SELECT command results in a READ-WRITE attribute in the last line, while the EXAMINE command gives you a READ-ONLY. That's precisely the difference between SELECT and EXAMINE: they essentially do the same thing, but EXAMINE selects the folder in read-only mode and doesn't let you make any changes.

Nice! Today we learned how to work with folders in IMAP. You can select a folder to work with using the SELECT (read-write) or EXAMINE (read-only) commands. Folders can be created with the CREATE command and deleted with the DELETE command (duh). Subfolders may be created by appending the hierarchy delimiter (e.g. ".") followed by the subfolder name to the parent folder name. All folders may be shown using the LIST command, while the LSUB command lists only subscribed folders. Folder subscription may be toggled using the SUBSCRIBE and UNSUBSCRIBE commands.

I hope this was interesting, and that you'll come back to learn more about IMAP in the upcoming articles! :)

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